The power density of power conversion circuitry (watts per cubic inch) is related to the energy storage capacity of passive components such as inductors and capacitors. The energy density, in joules/cubic inch, of inductors and capacitors requires increases in switching frequency to attain improvements in power density, transferring this stored energy through more charge/discharge cycles per second. As switching frequency increases, switching electronics and passive circuit components lose more energy to heat loss; therefore, at increasing frequencies, the efficiency of the system diminishes. Heat loss based on switching frequency places a hard limit on the possible power density of switching power conversion circuitry.
The foundations of electrical energy conversion lie in the ability to store energy within the magnetic field of an inductor or transformer. The ubiquitous switching power converter utilizes a transistor switch to endow a magnetic device element with potential energy in the form of a magnetic field. The switching power converter then releases the potential energy at a different voltage and current, thus performing an energy conversion or transformation.
Power converters are ideally small, light, efficient and inexpensive. Based on the physical properties of the components, existing power converter technology is nearing a physical limit. For a given energy density, the losses in a radio frequency (RF) inductor vary in proportion to the ratio of the wavelength of the operating frequency to the cube root of the volume of the inductor. Consequently, to maintain efficiency of the power converter, if the size of the inductive components is decreased, the switching frequency must be increased. However, increased switching frequency leads to greater losses due to heat in the switching components of the converter. Where the heat resulting from overall system losses cannot be removed from the limited surface area of the converter, no further reduction in size is possible. A practical limitation of switching frequency is in the low megahertz (MHz) range.
Capacitors have inherently higher energy density than inductors. Switching capacitor power conversion circuits can utilize capacitors to provide superior power density than inductor-based switching circuits. However, switching capacitor circuits can only provide small-integer voltage ratio transformations, limiting their utility to power conversion rather than voltage regulation.
State of the art power conversion technology utilizes a resonant piezoelectric structure excited from an alternating voltage to provide a voltage-step-up in a manner loosely analogous to that of a magnetic voltage transformer. Power converters utilizing piezoelectric structures generate high voltages for miniature fluorescent or electroluminescent lighting. Piezoelectric devices are inherently fragile, and therefore not suitable for many applications.
Consequently, it would be advantageous if an apparatus existed that is suitable for efficiently converting power at low frequencies with high energy density, and robust enough for most applications.